Semiconductor device

ABSTRACT

Related-art back bias generation circuits cause a problem where a long time is required for transition between an operating state and a standby state because driving power is lowered to reduce the power consumption in the standby state. A back bias generation circuit outputs a predetermined voltage. The predetermined voltage is the back bias voltage of a substrate in a standby mode. A bias control circuit stores an electrical charge while a circuit block is in an operating mode, supplies the stored electrical charge to the substrate of a MOSFET included in the circuit block when the circuit block transitions from the operating mode to the standby mode, and subsequently supplies the output of the back bias generation circuit to the substrate of the MOSFET.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2016-191234 filed onSep. 29, 2016 including the specification, drawings, and abstract isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to a semiconductor device. For example,the present invention relates to a semiconductor device capable ofcontrolling the back bias voltage of a MOSFET (Metal-Oxide-SemiconductorField-Effect Transistor).

There has been a known technology for controlling a back bias voltage ofa substrate in order to change the threshold voltage of a MOSFET asneeded to reduce a leakage current in a standby state.

SUMMARY

However, the driving power of a related-art circuit for generating theback bias voltage is lowered to reduce the power required for thestandby state. This causes a problem where a long time is required fortransition between an operating state and the standby state.

Other problems and novel features will become apparent from thefollowing description and from the accompanying drawings.

In an aspect of the present invention, a bias control circuit stores asupplied electrical charge while a circuit block is in an operatingmode. When the circuit block transitions from the operating mode to astandby mode, the bias control circuit supplies the stored electricalcharge to a MOSFET substrate included in the circuit block, and thensupplies the output of a voltage generation circuit to the MOSFETsubstrate.

The above aspect of the present invention reduces the time required fortransition between an operating state and a standby state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a semiconductordevice according to a first embodiment of the present invention;

FIG. 2 is a diagram illustrating a configuration of the semiconductordevice according to a second embodiment of the present invention;

FIG. 3A is a diagram illustrating a configuration of a bulk MOSFET, andFIG. 3B is a diagram illustrating a configuration of an SOI (Silicon onInsulator) MOSFET;

FIG. 4A is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of a bulk NMOS transistor, FIG.4B is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of a bulk PMOS transistor, FIG.4C is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of an SOI NMOS transistor, andFIG. 4D is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of an SOI PMOS transistor;

FIG. 5A is a diagram illustrating an exemplary relationship between asource-drain leakage current Ioff in the OFF state and back bias voltagevsub of an SOI NMOS transistor, and FIG. 5B is a diagram illustrating anexemplary relationship between the source-drain leakage current Ioff inthe OFF state and back bias voltage vsub of an SOI PMOS transistor;

FIG. 6 is a diagram illustrating an example of a back bias voltagecontrol method;

FIG. 7 is a diagram illustrating an example of back bias voltage controlin an operating state and standby state of a circuit block;

FIG. 8 is a diagram illustrating a configuration of a circuit includedin a VBBGEN(p);

FIGS. 9A and 9B are diagrams illustrating an operation of the VBBGEN(p);

FIG. 10 is a diagram illustrating a configuration of a circuit includedin a VBBGEN(n);

FIGS. 11A and 11B are diagrams illustrating an operation of theVBBGEN(n);

FIG. 12 is a diagram illustrating configurations of an N bias controlcircuit and P bias control circuit according to the second embodiment;

FIG. 13 is a timing diagram illustrating operations of the circuitsshown in FIG. 12;

FIG. 14 is a diagram illustrating a configuration of the N bias controlcircuit according to a third embodiment of the present invention;

FIG. 15 is a timing diagram illustrating an exemplary operation of thecircuit shown in FIG. 14;

FIG. 16 is a timing diagram illustrating another exemplary operation ofthe circuit shown in FIG. 14;

FIG. 17 is a diagram illustrating a bias control circuit according to afifth embodiment of the present invention;

FIG. 18 is a diagram illustrating temporal voltage changes that occur ina node vncapn when a capacitor common to all blocks is charged;

FIG. 19 is a diagram illustrating temporal voltage changes that occur ina node vncap when capacitors for individual functional blocks aresequentially charged;

FIG. 20 is a diagram illustrating an exemplary cross-sectional structureof a substrate for applying different bias voltages to variousfunctional blocks within a chip;

FIG. 21 is a diagram illustrating the bias control circuit according toa sixth embodiment of the present invention; and

FIG. 22 is a timing diagram illustrating an operation of the circuitshown in FIG. 21.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a diagram illustrating a configuration of a semiconductordevice 1 according to a first embodiment of the present invention.

A circuit block 2 has two operation states, namely, an operating modeand a standby mode. The circuit block 2 includes a MOSFET 4.

A back bias generation circuit 3 outputs a predetermined voltage. Thepredetermined voltage is a back bias voltage of a substrate in thestandby mode.

A bias control circuit 7 stores an electrical charge while the circuitblock 2 is in the operating mode. When the circuit block 2 transitionsfrom the operating mode to the standby mode, the bias control circuit 7supplies the stored electrical charge to the substrate of the MOSFET 4included in the circuit block 2, and then supplies the output of theback bias generation circuit 3 to the substrate of the MOSFET 4.

As described above, when the circuit block transitions from theoperating mode to the standby mode, the first embodiment first suppliesto the substrate of the MOSFET the electrical charge stored while thecircuit block is in the operating mode. As a result, the time requiredfor transition from an operating state to a standby state is shorterthan when the output of the back bias generation circuit is initiallysupplied to the substrate of the MOSFET.

Second Embodiment

FIG. 2 is a diagram illustrating a configuration of a semiconductordevice 10 according to a second embodiment of the present invention.

The semiconductor device 10 is a microcomputer capable of controllingthe back bias voltage. As illustrated in FIG. 2, the semiconductordevice 10 includes functional blocks such as a CPU (Central ProcessingUnit) core 12, a SRAM (Static Random Access Memory) 14, a flash memory16, a timer 18, and an interface 20. These functional blocks are mountedon an IC (Integrated Circuit) chip.

The semiconductor device 10 further includes a voltage regulator 22 anda back bias generation circuit 28.

The voltage regulator 22 receives a power supply voltage VIN from theoutside of the chip, converts the power supply voltage to an internalvoltage VDD, and supplies the internal voltage VDD to the functionalblocks. In the subsequent description, the voltage regulator 22 may bereferred to as the VDD supply source.

The back bias generation circuit 28 includes an NMOS back biasgeneration circuit (VBBGEN(n)) 24 and a PMOS back bias generationcircuit (VBBGEN(p)) 26.

The VBBGEN(n) 24 generates a back bias voltage (−VBB) from the internalvoltage VDD and supplies the generated back bias voltage (−VBB) to asubstrate PSUB for an NMOS transistor in each functional block.

The VBBGEN(p) 26 generates a back bias voltage (VDD+VBB) from theinternal voltage VDD and supplies the generated back bias voltage(VDD+VBB) to a substrate NSUB for a PMOS transistor in each functionalblock.

A threshold voltage VTH of the MOSFET can be changed in accordance witha back bias voltage vsub. For the NMOS transistor, the back bias voltagevsub is made negative to increase the absolute value of the thresholdvoltage VTH. For the PMOS transistor, the back bias voltage vsub isconversely made positive to increase the absolute value of the thresholdvoltage VTH.

FIG. 3A is a diagram illustrating a configuration of a bulk MOSFET.

As regards the bulk MOSFET, its source-drain terminal is coupled to thesubstrate by p-n junction. Therefore, an acceptable back bias voltagesetting is restricted to a range where no p-n junction forward currentflows.

FIG. 3B is a diagram illustrating a configuration of an SOI (Silicon onInsulator) MOSFET.

As regards the SOI MOSFET, its source-drain terminal is insulated fromthe substrate by a silicon oxide film layer. Therefore, the acceptableback bias voltage setting is not restricted to a range where no p-njunction forward current flows.

FIG. 4A is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of a bulk NMOS transistor. FIG.4B is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of a bulk PMOS transistor. FIG.4C is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of an SOI NMOS transistor. FIG.4D is a diagram illustrating the relationship between the back biasvoltage vsub and threshold voltage VTH of an SOI PMOS transistor.

As illustrated in FIGS. 4A to 4D, a comparison between the SOI and thebulk shows that the former is larger in the amount of threshold voltageVTH change relative to the back bias voltage vsub, that is, indVTH/dvsub, and is capable of changing the back bias voltage vsub by alarger amount. Thus, as a whole, the SOI is capable of changing thethreshold voltage VTH by a larger amount than the bulk.

FIG. 5A is a diagram illustrating an exemplary relationship between asource-drain leakage current Ioff in the OFF state and back bias voltagevsub of the SOI NMOS transistor.

FIG. 5B is a diagram illustrating an exemplary relationship between thesource-drain leakage current Ioff in the OFF state and back bias voltagevsub of an SOI PMOS transistor.

As regards the MOSFET, a dominant factor for causing a leakage currentis a subthreshold leakage current. For the NMOS transistor, the leakagecurrent Ioff can be decreased by making the back bias voltage vsubnegative. For the PMOS transistor, the leakage current Ioff can bedecreased by making the back bias voltage vsub positive.

Here, the leakage current Ioff changes exponentially relative to changesin the back bias voltage vsub. Therefore, the amount of change in theleakage current Ioff increases when the back bias voltage vsub is in thevicinity of 0 V, and decreases with an increase in the back bias voltagevsub.

FIG. 6 is a diagram illustrating an example of a back bias voltagecontrol method. An NMOS transistor N1 and a PMOS transistor P1 areformed in a pWell and an nWell, respectively. The pWell is coupled to adedicated back bias generation circuit (VBBGEN(n)) 24, and the nWell iscoupled to a dedicated back bias generation circuit (VBBGEN(p)) 26.

The back bias voltage of the NMOS transistor N1 and the back biasvoltage of the PMOS transistor P1 simultaneously change.

The threshold voltage VTH of the PMOS transistor P1 and the thresholdvoltage VTH of the NMOS transistor N1 are set to be low when the circuitblock in the semiconductor device is operating, that is, in theoperating state. This increases a switching speed to achieve high-speedoperation. Meanwhile, threshold voltage VTH of the PMOS transistor P1and the threshold voltage VTH of the NMOS transistor N1 are set to behigh when the circuit block in the semiconductor device is stopped, thatis, in the standby state. This decreases the leakage current to providelow power consumption. When the back bias voltage is dynamicallycontrolled as described above, high-speed operation and low powerconsumption can be both achieved.

FIG. 7 is a diagram illustrating an example of back bias voltage controlin the operating state and standby state of the circuit block. It isassumed that the power supply voltage of the circuit block is VDD.

When, in the operating state, the back bias voltage of the NMOStransistor N1 is set to 0 V and the back bias voltage of the PMOStransistor P1 is set to VDD, the absolute value of the threshold voltageVTH of the NMOS transistor and the absolute value of the thresholdvoltage VTH of the PMOS transistor both decrease. This increases an ONcurrent to provide high-speed operation.

Next, in the standby state, the back bias voltage of the NMOS transistorN1 is set to (−VBB), and the back bias voltage of the PMOS transistor P1is set to (VDD+VBB).

The above causes an increase in the absolute value of the thresholdvoltage VTH of the NMOS transistor and in the absolute value of thethreshold voltage VTH of the PMOS transistor. As this decreases theleakage current, the current consumption in the standby state can bereduced.

In order to provide practical control of the back bias voltage, however,the back bias voltage needs to be rapidly changed to reduce the timerequired for transition between the operating state and the standbystate. A MOSFET substrate can be regarded as a capacitor circuit nomatter whether it is a bulk or an SOI. Thus, the substrate needs to becharged and discharged at high speed in order to reduce the transitiontime. For such high-speed charging and discharging, it is essential thatthe driving power of the back bias generation circuit be sufficientlyhigh relative to the capacitance of the substrate.

In general, when the back bias voltage vnsub of the NMOS transistor isto be made negative by using a circuit operating on a single powersupply voltage VDD, a charge pump circuit or other DC-DC converter isused to generate the back bias voltage vnsub from VDD. At the same time,the back bias voltage vpsub of the PMOS transistor needs to be equal toor higher than VDD. The back bias voltages vnsub, vpsub are generated byusing a DC-DC converter. Therefore, it is necessary to increase thedriving power of the DC-DC converter. When, for example, a charge pumpcircuit, which is a kind of DC-DC converter, is used to increase thedriving power, it is necessary to raise the switching frequency orincrease the pump capacity. However, these two methods both increase thepower consumption of the charge pump circuit. When the power consumptionin the standby state is to be reduced by controlling the back biasvoltage, an increase in the power consumption of the charge pump circuitexerts an adverse effect. Consequently, related-art back bias generationcircuits cause a problem where a longtime is required for transitionbetween the operating state and the standby state because the drivingpower is lowered to reduce the power consumption in the standby state.

The above problem can be addressed by the present embodiment.

FIG. 8 is a diagram illustrating a configuration of a circuit includedin the VBBGEN(p) 26.

The VBBGEN(p) 26 is a general charge pump circuit. The VBBGEN(p) 26generates a positive voltage (VDD+VBB) from an input voltage VDD andoutputs the positive voltage (VDD+VBB) from an output terminal VOUT1.

The VBBGEN(p) 26 includes switches SWC11, SWC12, SWC13, capacitors C11,C12, and a VDD supply source 22 for supplying the voltage VDD.

The switch SWC11 couples a node ND11 to either a terminal g or aterminal v. The terminal g is coupled to a ground GND. The terminal v iscoupled to a node ND14. The capacitor C11 is disposed between the nodeND11 and a node ND12. The switch SWC12 opens or closes a path betweenthe node ND12 and the node ND14. The VDD supply source 22 is disposedbetween the node ND14 and the ground GND. The switch SWC13 opens orcloses a path between the node ND12 and a node ND13. The capacitor C12is disposed between the node ND13 and the ground GND. The node ND13 iscoupled to the output terminal VOUT1.

FIGS. 9A and 9B are diagrams illustrating an operation of the VBBGEN(p)26.

First of all, as illustrated in FIG. 9A, the switch SWC11 is set to g,the switch SWC12 is turned on, and the switch SWC13 is turned off. Thisstores the voltage VDD across the capacitor C11.

Next, as illustrated in FIG. 9B, the switch SWC11 is set to v, theswitch SWC12 is turned off, and the switch SWC13 is turned on. Thevoltage VDD is then developed at a negative terminal (−) of thecapacitor C11. Thus, the voltage at a positive terminal (+) of thecapacitor C11 is 2*VDD. That is to say, the output terminal VOUT1outputs a voltage that is two times the input voltage VDD.

FIG. 8 illustrates a circuit that generates a voltage of 2*VDD from theinput voltage VDD. When VBB>VDD, a high voltage, such as 3*VDD or 4*VDD,can be generated by coupling plural pieces of this circuit in series.When VBB<VDD, a desired voltage of VBB+VDD can be generated bydecreasing the output voltage of this circuit.

FIG. 10 is a diagram illustrating a configuration of a circuit includedin the VBBGEN(n) 24.

The VBBGEN(n) 24 is a general charge pump circuit. The VBBGEN (n) 24generates a negative voltage (−VBB) from an input voltage VDD andoutputs the negative voltage (−VBB) from an output terminal VOUT2.

The VBBGEN(n) 24 includes switches SWC21, SWC22, SWC23, capacitors C21,C22, and a VDD supply source 22 for supplying the voltage VDD.

The switch SWC21 couples a node ND21 to either a terminal g or aterminal v. The terminal g is coupled to the ground GND. The terminal vis coupled to the VDD supply source 22. The capacitor C21 is disposedbetween a node ND21 and a node ND22. The switch SWC22 opens or closes apath between the node ND22 and the ground GND. The VDD supply source 22is disposed between the terminal v and the ground GND. The switch SWC23opens or closes a path between the node ND22 and a node ND23. Thecapacitor C22 is disposed between the node ND23 and the ground GND. Thenode ND23 is coupled to the output terminal VOUT2.

FIGS. 11A and 11B are diagrams illustrating an operation of theVBBGEN(n) 24.

First of all, as illustrated in FIG. 11A, the switch SWC21 is set to v,the switch SWC22 is turned on, and the switch SWC23 is turned off. Thisstores the voltage VDD across the capacitor C21.

Next, as illustrated in FIG. 11B, the switch SWC21 is set to g, theswitch SWC22 is turned off, and the switch SWC23 is turned on. Thevoltage at a positive terminal of the capacitor C21 is then 0 V. Thus,the voltage at a negative terminal of the capacitor C21 is (−VDD). Thatis to say, the output terminal VOUT2 outputs a voltage having anopposite sign to the input voltage VDD.

FIG. 10 illustrates a circuit that generates a voltage of *VDD from theinput voltage VDD. When VBB>VDD, a high voltage, such as (−2)*VDD or(−3)*VDD, can be generated by coupling plural pieces of this circuit inseries. When VBB<VDD, a desired voltage of −VBB can be generated bydecreasing the output voltage of this circuit.

FIG. 12 is a diagram illustrating configurations of an N bias controlcircuit NC1 and P bias control circuit PC1 according to the secondembodiment.

FIG. 12 shows a PMOS transistor P1 and an NMOS transistor N1, whichrespectively represent plural PMOS transistors and NMOS transistorsincluded in plural circuit blocks of the semiconductor device.

The N bias control circuit NC1 is disposed between the VBBGEN (n) 24 andthe substrate PSUB of the NMOS transistor N1. The P dial control circuitPC1 is disposed between the VBBGEN(p) 26 and the substrate NSUB of thePMOS transistor P1. The N bias control circuit NC1 and the P biascontrol circuit PC1 simultaneously operate.

The N bias control circuit NC1 includes a switch SWN1, a capacitor NCAP,and a switch SWN2.

The switch SWN1 couples a node vncap to either a terminal a or aterminal b. The terminal a is open. The terminal b is coupled to theoutput of the NMOS back bias generation circuit 24.

The capacitor NCAP is disposed between the node vncap and the groundGND.

The switch SWN2 is couples the substrate PSUB to one of a terminal p, aterminal q, and a terminal r. The terminal p is coupled to the output ofthe VBBGEN(n) 24. The terminal q is coupled to the node vncap. Theterminal r is coupled to the ground GND.

The P bias control circuit PC1 includes a switch SWP1, a capacitor PCAP,and a switch SWP2.

The switch SWP1 couples a node vpcap to either the terminal a or theterminal b. The terminal a is open. The terminal b is coupled to theoutput of the PMOS back bias generation circuit 26.

The capacitor PCAP is disposed between the node vpcap and the groundGND.

The switch SWP2 couples the substrate NSUB to one of the terminal p, theterminal q, and the terminal r. The terminal p is coupled to the outputof the PMOS back bias generation circuit 26. The terminal q is coupledto the node vpcap. The terminal r is coupled to the ground GND.

For the sake of simplicity, it is assumed that the capacitance of thecapacitor PCAP is equal to the parasitic capacitance of the substrateNSUB, and that the capacitance of the capacitor NCAP is equal to theparasitic capacitance of the substrate PSUB. The voltages of thesubstrate PSUB and substrate NSUB can be rapidly changed by thecapacitor NCAP and the capacitor PCAP.

FIG. 13 is a timing diagram illustrating operations of the circuitsshown in FIG. 12.

It is assumed that the output voltage of the VBBGEN(n) 24 is always−VBB, and that the output voltage of the VBBGEN (p) 26 is always(VDD+VBB).

It is assumed that, in an initial state at time t0, the circuit blocksin the semiconductor device are in the operating state, and that thevoltages of the substrate PSUB and capacitor NCAP are both equal to aground voltage of 0 V, and further that the voltages of the substrateNSUB and capacitor PCAP are both equal to the power supply voltage VDD.

When the CPU core 12 sets the switch SWN1 and switch SWP1 to b, theVBBGEN (n) 24 is coupled to the capacitor NCAP, and the VBBGEN (p) 26 iscoupled to the capacitor PCAP. When the CPU core 12 sets the switch SWN2and switch SWP2 to r, the substrate PSUB is coupled to the ground GND,and the substrate NSUB is coupled to the power supply source VDD. As aresult, the capacitor NCAP is gradually charged to −VBB although thevoltage of the substrate PSUB remains at 0 V, and the capacitor PCAP isgradually charged to (VDD+VBB) although the voltage of the substrateNSUB remains at VDD. The time required for such charging is governed bythe maximum output current of the VBBGEN(n) 24 and VBBGEN(p) 26.

When, at time t1, the voltage of the capacitor NCAP is −VBB and thevoltage of the capacitor PCAP is (VDD+VBB), the VBBGEN(n) 24 and theVBBGEN(p) 26 automatically stop to terminate charging.

At time t2, the semiconductor device receives, from the outside, atransition instruction for transitioning from the operating mode to thestandby mode. In this instance, the CPU core 12 sets the switch SWN1 andthe switch SWP1 to a, and sets the switch SWN2 and the switch SWP2 to q.This uncouples the VBBGEN(n) 24 from the capacitor NCAP, uncouples theVBBGEN(p) 26 from the capacitor PCAP, couples the substrate PSUB to thecapacitor NCAP, and couples the substrate NSUB to the capacitor PCAP.The substrate PSUB is then rapidly charged by a negative electricalcharge stored in the capacitor NCAP so that the substrate PSUB is at thesame voltage (−½*VBB) as the capacitor NCAP. Further, the substrate NSUBis rapidly charged by a positive electrical charge stored in thecapacitor PCAP so that the substrate NSUB is at the same voltage(VDD+½*VBB) as the capacitor PCAP. The time required for such anoperation is determined by the on-resistance of the switch SWN2 andswitch SWP2 and the capacitance of the substrate PSUB, capacitor NCAP,substrate NSUB, and capacitor PCAP. However, the substrate PSUB and thesubstrate NSUB can be charged more rapidly by sufficiently decreasingthe on-resistance of the switch SWN2 and switch SWP2 than when thesubstrate PSUB is charged by the VBBGEN(n) 24 and the substrate NSUB ischarged by the VBBGEN(p) 26. In this instance, the voltage of thesubstrate PSUB is changed to a value half a target voltage (−½*VBB), andthe voltage of the substrate NSUB is changed to a value half a targetvoltage (VDD+½*VBB). However, such change amounts will provide asufficient effect of leakage current reduction because the substratevoltage dependence of a MOSFET leakage current exhibits the greatestchange in the vicinity of 0 V as mentioned earlier.

When, at time t3, the CPU core 12 further sets the switch SWN1 andswitch SWP1 to a and sets the switch SWN2 and switch SWP2 to p, theVBBGEN(n) 24 is coupled to the substrate PSUB, and the VBBGEN(p) 26 iscoupled to the substrate NSUB. As a result, the substrate PSUB isgradually charged to (−VBB), and the substrate NSUB is gradually chargedto (VDD+VBB).

When, at time t4, the voltage of the substrate PSUB is (−VBB) and thevoltage of the substrate NSUB is (VDD+VBB), the VBBGEN(n) 24 and theVBBGEN(p) 26 automatically stop to terminate charging. The circuitblocks are then placed in the standby state.

At time t5, the semiconductor device receives, from the outside, atransition instruction for transitioning from the operating mode to thestandby mode. The CPU core 12 sets the switch SWN1 and the switch SWP1to a, and sets the switch SWN2 and the switch SWP2 to q. This recouplesthe substrate PSUB to the capacitor NCAP, and recouples the substrateNSUB to the capacitor PCAP. The negative electrical charge of thesubstrate PSUB is then rapidly regenerated to the capacitor NCAP so thatthe substrate PSUB is at the same voltage (−¾*VBB) as the capacitorNCAP. Further, the positive electrical charge in the substrate NSUB israpidly regenerated to the capacitor PCAP so that the substrate NSUB isat the same voltage (VDD+¾*VBB) as the capacitor PCAP. The time requiredfor such a regeneration operation is determined by the on-resistance ofthe switch SWN2 and switch SWP2 and the capacitance of the substratePSUB, substrate NSUB, capacitor NCAP, and capacitor PCAP. Therefore, theregeneration operation is performed at the same high speed as theoperation performed at time t2.

At time t6, the CPU core 12 further sets the switch SWN1 and switch SWP1to b and sets the switch SWN2 and switch SWP2 to r.

The substrate PSUB is then rapidly discharged to 0 V, and the substrateNSUB is rapidly discharged to VDD. The time required for this dischargeoperation is determined by the on-resistance of the switch SWN2 andswitch SWP2 and the capacitance of the substrate PSUB and substrateNSUB. Therefore, the discharge operation is also performed at highspeed. Further, the capacitor NCAP is recoupled to the VBBGEN(n) 24 andgradually charged to (−VBB), and the capacitor PCAP is recoupled to theVBBGEN(p) 26 and gradually charged to (VDD+VBB).

When, at time t7, the substrate PSUB is discharged to 0 V, and thesubstrate NSUB is discharged to VDD, the circuit blocks are placed inthe operating state.

When, at time t8, the voltage of the capacitor NCAP is (−VBB), and thevoltage of the capacitor PCAP is (VDD+VBB), the VBBGEN(n) 24 and theVBBGEN(p) 26 automatically stop to terminate charging.

The state at time t8 is the same as the state at time t1. Subsequently,a cycle formed of states at time t1 to time t8 is repeated forprocessing purposes.

At time t5, the substrate PSUB regenerates an electrical charge to thecapacitor NCAP, and the substrate NSUB regenerates an electrical chargeto the capacitor PCAP. The present embodiment will now be compared witha case where electrical power generated, for example, by a motor isregenerated to a capacitor. In this case, as the motor generateselectromotive force, an electrical charge can be regenerated even ifthere is no potential difference between the motor and the capacitor. Inthe present embodiment, however, the substrate PSUB and the substrateNSUB are passive elements and do not generate electromotive force.Therefore, the present embodiment uncouples the substrate PSUB from thecapacitor NCAP and the substrate NSUB from the capacitor PCAP during aperiod between time t3 and time t5 in order to generate a potentialdifference between the substrates and the capacitors. In this manner,the present embodiment achieves regeneration.

As described above, in order to charge the substrate PSUB and thesubstrate NSUB during transition to the standby state at time t2, thepresent embodiment uses the capacitor NCAP and capacitor PCAP and notthe VBBGEN(n) 24 and VBBGEN(p) 26. Therefore, charging can be rapidlyaccomplished without regard to the current drive capability of theVBBGEN(n) 24 and VBBGEN(p) 26. Further, at time t5, the electricalcharge of the substrate PSUB is regenerated to the capacitor NCAP, andthe electrical charge of the substrate NSUB is regenerated to thecapacitor PCAP. This reduces the power consumption of the VBBGEN(n) 24and VBBGEN(p) 26.

Electrical power of the VBBGEN(n) 24 will now be described.

When the VBBGEN(n) 24 is assumed to be a general charge pump circuit,the internal voltage of the VBBGEN(n) 24 remains at (−VBB) irrespectiveof the voltage of a charge target (e.g., the capacitor NCAP). Therefore,an energy equivalent to the difference between the internal voltage andthe voltage of the capacitor NCAP is consumed by the switch SWN1, whichcouples the VBBGEN(n) 24 to the capacitor NCAP. If, for example, thecapacitance of the capacitor NCAP is Ccap in a case where the capacitorNCAP is to be charged from 0 V to (−VBB), the electrical power P1outputted from the VBBGEN(n) 24 and the electrical power P2 stored inthe capacitor NCAP are expressed by the equation below.P1=Ccap*VBB ²P2=(½)*Ccap*VBB ²

The electrical power outputted from the VBBGEN(n) 24 is two times theelectrical power stored in the capacitor NCAP. Half the electrical poweroutputted from the VBBGEN(n) 24 is consumed by the wiring resistancebetween the capacitor NCAP and the VBBGEN(n) 24.

It is assumed that the power efficiency Effb of the VBBGEN(n) 24 isconstant, namely, its internal loss is proportional to its electricalpower output. The power consumption of the VBBGEN(n) 24 operating asindicated in the timing diagram of FIG. 13 is P3 between time t0 andtime t1, P4 between time t3 and time t4, and P5 between time t6 and timet8.P3=Ccap*VBB ² /EffbP4=(½)*Ccap*VBB ² /EffbP5=(¼)*Ccap*VBB ² /Effb

That is to say, the present embodiment consumes an electrical power ofP3 during an initial interval between time t0 and time t1, but consumesan electrical power of P6 during a subsequent cycle between time t1 andtime t8.P6=(¾)*Ccap*VBB ² /Effb

Meanwhile, a related-art method consumes an electrical power of P3during each cycle. Therefore, the power consumption of the VBBGEN(n) 24according to the present embodiment is reduced to three-fourths thepower consumption during the use of the related-art method. The reasonis that a regenerative operation is performed at time t5.

The power consumption of the VBBGEN(p) 26 can also be reduced tothree-fourths the power consumption during the use of the related-artmethod.

Third Embodiment

Although the following only describes how the back bias voltage of theNMOS transistor N1 is controlled, the back bias voltage of the PMOStransistor P1 can be controlled in the same manner as described below.

FIG. 14 is a diagram illustrating a configuration of the N bias controlcircuit NC1 according to a third embodiment of the present invention.

FIG. 14 shows an NMOS transistor N1 that represents plural NMOStransistors included in plural circuit blocks of the semiconductordevice.

The N bias control circuit NC1 is disposed between the VBBGEN (n) 24 andthe substrate PSUB of the NMOS transistor N1.

The N bias control circuit NC1 includes the switch SWN1, the capacitorNCAP, the switch SWN2, and a switch SWN3.

The switch SWN1 couples the node vncap to one of the terminal a, theterminal b, and a terminal c. The terminal a is open. The terminal b iscoupled to the output of the VBBGEN (n) 24. The terminal c is coupled tothe ground GND.

The capacitor NCAP is disposed between the node vncap and a node vncapb.

The switch SWN2 is couples the substrate PSUB to one of the terminal p,the terminal q, and the terminal r. The terminal p is coupled to theoutput of the VBBGEN(n) 24. The terminal q is coupled to the node vncap.The terminal r is coupled to the ground GND.

The switch SWN3 couples the node vncapb to either a terminal x or aterminal y. The terminal x is coupled to the VDD supply source 22. Theterminal y is coupled to the ground GND.

For the sake of simplicity, it is assumed that the capacitance of thecapacitor NCAP is equal to the parasitic capacitance of the substratePSUB. The voltage of the node vncapb is allowed to transition betweenthe ground GND and the VDD in order to let the capacitor NCAP functionas a charge pump in place of the VBBGEN(n) 24.

FIG. 15 is a timing diagram illustrating an exemplary operation of thecircuit shown in FIG. 14.

It is assumed that the output voltage (−VBB) of the VBBGEN(n) 24 isequal to (−VDD).

It is assumed that, in an initial state at time t0, the circuit blocksin the semiconductor device are in the operating state, and that thevoltages of the substrate PSUB and capacitor NCAP are both equal to aground voltage of 0 V.

When the CPU core 12 sets the switch SWN1 to c, the switch SWN2 to r,and the switch SWN3 to x, the capacitor NCAP is coupled to the VDDsupply source 22, and the substrate PSUB is coupled to the ground GND.This causes the voltage of the substrate PSUB to remain at 0 V, butgradually charges the capacitor NCAP to VDD. The time required for suchcharging is governed by the maximum output current of the VBBGEN(n) 24and VBBGEN(p) 26. The time required for such charging is governed by theoutput current of the VDD supply source 22. In general, thepositively-biased VDD supply source 22 is higher than thenegatively-biased VBBGEN(n) 24 in current supply capacity and powerefficiency. Therefore, the present embodiment is capable of charging thecapacitor NCAP at higher speed and lower power consumption than thesecond embodiment.

When, at time t1, the voltage of the node vncapb is VDD, chargingterminates.

At time t2, the CPU core 12 switch sets the switch SWN1 to a, the switchSWN2 to r, and the switch SWN3 to y. This sets the voltage of the nodevncapb to 0 V and the voltage of the node vncap to (−VBB) whilemaintaining the electrical charge of the capacitor NCAP.

At time t3, the semiconductor device receives, from the outside, atransition instruction for transitioning from the operating mode to thestandby mode. In this instance, the CPU core 12 sets the switch SWN1 toa, the switch SWN2 to q, and the switch SWN3 to y. This couples thesubstrate PSUB to the capacitor NCAP. The negative electrical charge ofthe capacitor NCAP then rapidly charges the substrate PSUB so that thevoltages of the substrate PSUB and capacitor NCAP are (−½)*VBB. The timerequired for such charging is determined by the on-resistance of theswitch SWN2 and the capacitance of the substrate PSUB and capacitorNCAP. However, when the on-resistance of the switch SWN2 is decreased toa sufficiently low level, the substrate PSUB can be charged at higherspeed than when it is charged by the VBBGEN(n). In this instance, thevoltage of the substrate PSUB is changed to a value half a targetvoltage, that is, (−½) *VBB. However, such a change amount will providea sufficient effect of leakage current reduction because the substratePSUB voltage dependence of a MOSFET leakage current exhibits thegreatest change in the vicinity of 0 V.

When, at time t4, the CPU core 12 sets the switch SWN1 to a, the switchSWN2 to p, and the switch SWN3 to y, the VBBGEN(n) 24 is coupled to thesubstrate PSUB. The substrate PSUB is then gradually charged to (−VBB).

When, at time t5, the voltage of the substrate PSUB is equal to VBB, theVBBGEN(n) 24 automatically stops to terminate charging. This places thecircuit blocks in the standby state.

At time t6, the semiconductor device receives, from the outside, atransition instruction for transitioning from the standby mode to theoperating mode. When the CPU core 12 sets the switch SWN1 to a, theswitch SWN2 to q, and the switch SWN3 to y, the substrate PSUB isrecoupled to the capacitor NCAP. The negative electrical charge of thesubstrate PSUB is then rapidly regenerated to the capacitor NCAP so thatthe substrate PSUB and the capacitor NCAP are at the same voltage, thatis, (−¾)*VBB. The time required for such a regeneration operation isdetermined by the on-resistance of the switch SWN2 and the capacitanceof the substrate PSUB and capacitor NCAP. Therefore, the regenerationoperation is performed at the same high speed as the operation performedat time t2.

At time t7, the CPU core 12 further sets the switch SWN1 to c, theswitch SWN2 to r, and the switch SWN3 to x. The substrate PSUB is thenrapidly discharged to 0 V. The time required for this dischargeoperation is determined by the on-resistance of the switch SWN2 and thecapacitance of the substrate PSUB. Therefore, the discharge operation isalso performed at high speed. Further, the node vncap is coupled to theground GND, and the node vncapb is coupled to the VDD supply source 22and charged again to VDD.

When, at time t8, the substrate PSUB is discharged to 0 V, the circuitblocks in the semiconductor device are placed in the operating state.

When, at time t9, the voltage of the node vncapb is VDD, charging of thecapacitor NCAP terminates.

At time t10, the CPU core 12 further sets the switch SWN1 to a, theswitch SWN2 to r, and the switch SWN3 to y. This again sets the voltageof the node vncapb to 0 V and the voltage of the node vncap to (−VBB)while maintaining the electrical charge of the capacitor NCAP.

The state at time t10 is the same as the state at time t2. Subsequently,a cycle formed of states at time t2 to time t10 is repeated forprocessing purposes.

Electrical power of the back bias voltage will now be described, as isthe case with the second embodiment. In this instance, the electricalpower of the VDD supply source 22 is taken into consideration inaddition to the electrical power of the VBBGEN(n) 24. The VDD supplysource 22, which supplies a positive bias, is higher in efficiency thanthe VBBGEN(n). Here, it is assumed that the efficiency of the VDD supplysource 22 is Effd. It should be noted that Effd>Effb.

The power consumption of the VBBGEN (n) 24 and VDD supply source 22during an operation indicated in the timing diagram of FIG. 15 is P7between time t0 and time t1, P8 between time t4 and time t5, and P9between time t7 and time t9.P7=Ccap*VDD ² /EffdP8=(½)*Ccap*VBB ² /EffbP9=(¼)*Ccap*VDD ² /Effd

That is to say, as Effd>Effb and VDD=VBB, P7 is lower than P1 in thesecond embodiment during an initial interval between time t0 and timet1, and the power consumption P10 is lower than P6 in the secondembodiment during a subsequent cycle between time t2 and time t10.P10=Ccap*VBB ²*((½)*Effb+(¼)*Effd)

As described above, the present embodiment achieves charging by using ahighly efficient VDD supply source. Thus, the present embodiment reducesthe power consumption to a lower level than the second embodiment.

Fourth Embodiment

FIG. 16 is a timing diagram illustrating another exemplary operation ofthe circuit shown in FIG. 14.

Here, it is assumed that the output voltage (−VBB) of the VBBGEN(n) 24is equal to (−VDD), as is the case with the third embodiment.

At time t0, the circuit blocks in the semiconductor device are assumedto be in the operating state, and the voltages of the substrate PSUB andcapacitor NCAP are assumed to be both equal to a ground voltage of 0 V.

When the CPU core 12 sets the switch SWN1 to c, the switch SWN2 to r,and the switch SWN3 to x, the VDD supply source 22 is coupled to thecapacitor NCAP, and the substrate PSUB is coupled to the ground GND.This causes the voltage of the substrate PSUB to remain at 0 V, butgradually charges the node vncapb to VDD. The time required for suchcharging is governed by the maximum output current of the VDD supplysource 22. In general, the positively-biased VDD supply source 22 ishigher than the negatively-biased VBBGEN(n) in current supply capacityand power efficiency. Therefore, the present embodiment is capable ofcharging the capacitor NCAP at higher speed and lower power consumptionthan the second embodiment.

When, at time t1, the voltage of the node vncapb is VDD, chargingterminates.

At time t2, the CPU core 12 switch sets the switch SWN1 to b. Thischarges the node vncap to (−VBB).

When, at time t3, the voltage of the node vncap is (−VBB), chargingterminates. As a result, a voltage of 2*VDD is stored across thecapacitor NCAP, that is, between the node vncapb and the node vncap.

At time t4, the CPU core 12 couples the switch SWN1 to a and couples theswitch SWN3 to y. As a result, the voltage of the node vncap is(−2*VBB).

At time t5, the semiconductor device receives, from the outside, atransition instruction for transitioning from the operating mode to thestandby mode. In this instance, the CPU core 12 couples the switch SWN1to a, couples the switch SWN2 to q, and couples the switch SWN3 to y.This couples the substrate PSUB to the capacitor NCAP. The negativeelectrical charge of the capacitor NCAP then rapidly charges thesubstrate PSUB so that the substrate PSUB and the capacitor NCAP are atthe same voltage, that is, (−VBB). The time required for such a chargingoperation is determined by the on-resistance of the switch SWN2 and thecapacitance of the substrate PSUB and capacitor NCAP. However, when theon-resistance of the switch SWN2 is decreased to a sufficiently lowlevel, the substrate PSUB can be charged at higher speed than when it ischarged by the VBBGEN (n) 24. In marked contrast to the second and thirdembodiments, the present embodiment is capable of charging the substratePSUB to (−VBB) by using only an electrical charge from the capacitorNCAP. This further increases the speed of possible voltage change in thesubstrate PSUB. As a result, the present embodiment produces asignificant leakage current reduction effect.

When, at time t6, charging terminates, the CPU core 12 couples theswitch SWN1 to a, couples the switch SWN2 to p, and couples the switchSWN3 to y. This uncouples the substrate PSUB from the capacitor NCAP andcouples the substrate PSUB to the VBBGEN(n) 24.

At time t7, the semiconductor device receives, from the outside, atransition instruction for transitioning from the standby mode to theoperating mode. The CPU core 12 then couples the switch SWN1 to b,couples the switch SWN2 to r, and couples the switch SWN3 to x. Thisrapidly discharges the substrate PSUB to 0 V while at the same timecharging the node vncapb to VDD.

When, at time t8, the substrate PSUB is discharged to 0 V, the circuitblocks in the semiconductor are placed in the operating state.

At time t9, the node vncapb is charged to VDD.

At time t10, the CPU core 12 couples the switch SWN1 to a, couples theswitch SWN2 to r, and couples the switch SWN3 to y. This again sets thevoltage of the node vncapb to 0 V and the voltage of the node vncap to(−2*VBB) while maintaining the electrical charge of the capacitor NCAP.

The state at time t10 is the same as the state at time t4. Subsequently,a cycle formed of states at time t4 to time t10 is repeated forprocessing purposes.

In marked contrast to the second and third embodiments, the presentembodiment stores two times the VDD voltage in the capacitor NCAP attime t5, and thus is capable of changing the voltage of the substratePSUB to (−VBB) by using only the electrical charge of the capacitorNCAP. As a result, the present embodiment produces a greater leakagecurrent reduction effect than the second and third embodiments.

Fifth Embodiment

FIG. 17 is a diagram illustrating a bias control circuit according to afifth embodiment of the present invention.

In the fifth embodiment, a bias control circuit having a capacitor isprovided for each functional block in a semiconductor chip. Thefunctional blocks are, for example, the CPU 12, the SRAM 14, the flashmemory 16, and the timer 18.

The semiconductor device includes bias control circuits NX1, NX2, NX3,NXn for the CPU core 12, the SRAM 14, the flash memory 16, and the timer18, respectively.

The bias control circuit NX1 includes a switch SWN11, a capacitorNCAP11, and a switch SWN21.

The switch SWN11 couples a node vncap1 to either the terminal a or theterminal b. The terminal a is open. The terminal b is coupled to theoutput of the VBBGEN(n) 24. The capacitor NCAP11 is disposed between thenode vncap1 and the ground GND. The switch SWN21 couples the substratePSUB of the NMOS transistor in the SRAM 14 to one of the terminal p, theterminal q, and the terminal r. The terminal p is coupled to the outputof the VBBGEN(n) 24. The terminal q is coupled to the node vncap1. Theterminal r is coupled to the ground GND.

The bias control circuit NX2 includes a switch SWN12, a capacitorNCAP12, and a switch SWN22.

The switch SWN12 couples a node vncap2 to either the terminal a or theterminal b. The terminal a is open. The terminal b is coupled to theoutput of the VBBGEN(n) 24. The capacitor NCAP12 is disposed between thenode vncap2 and the ground GND. The switch SWN22 couples the substratePSUB of the NMOS transistor in the CPU core 12 to one of the terminal p,the terminal q, and the terminal r. The terminal p is coupled to theoutput of the VBBGEN(n) 24. The terminal q is coupled to the nodevncap2. The terminal r is coupled to the ground GND.

The bias control circuit NX3 includes a switch SWN13, a capacitorNCAP13, and a switch SWN23.

The switch SWN13 couples a node vncap3 to either the terminal a or theterminal b. The terminal a is open. The terminal b is coupled to theoutput of the VBBGEN(n) 24. The capacitor NCAP13 is disposed between thenode vncap3 and the ground GND. The switch SWN23 couples the substratePSUB of the NMOS transistor in the flash memory 16 to one of theterminal p, the terminal q, and the terminal r. The terminal p iscoupled to the output of the VBBGEN(n) 24. The terminal q is coupled tothe node vncap3. The terminal r is coupled to the ground GND.

The bias control circuit NXn includes a switch SWN1 n, a capacitor NCAP1n, and a switch SWN2 n.

The switch SWN1 n couples a node vncapn to either the terminal a or theterminal b. The terminal a is open. The terminal b is coupled to theoutput of the VBBGEN (n) 24. The capacitor NCAP1 n is disposed betweenthe node vncapn and the ground GND. The switch SWN2 n couples thesubstrate PSUB of the NMOS transistor in the flash memory 16 to one ofthe terminal p, the terminal q, and the terminal r. The terminal p iscoupled to the output of the VBBGEN(n) 24. The terminal q is coupled tothe node vncapn. The terminal r is coupled to the ground GND.

The VBBGEN (n) 24 is commonly provided for the functional blocks 16, 12,14, 18 to output a bias voltage (−VBB).

Each of the bias control circuits NX1-NXn controls the associatedcircuit block in a manner described in conjunction with the second,third, or fourth embodiment. Such control includes at least theprocesses described below.

When the associated circuit block is selected, the bias control circuitsNX1-NXn each store an electrical charge supplied from the VBBGEN (n) 24while the associated circuit block is in the operating mode. When theassociated circuit block transitions from the operating mode to thestandby mode, the bias control circuits NX1-NXn each supply the storedelectrical charge to the substrate of a MOSFET included in theassociated circuit block, and then cause the VBBGEN(n) 24 to supply itsoutput to the substrate. The bias control circuits NX1-NXn are assigneddifferent priorities and selected according to the assigned priorities.

If plural power supply domains are included in the chip and can beturned on/off and set to different power supply voltage settings, it ispreferable that each power supply domain be provided with its owncapacitor.

Referring to FIG. 13, the capacitors NCAP, PCAP are charged by theVBBGEN(n) 24 and the VBBGEN(p) 26 during a period between time t0 andtime t1. However, a significant amount of time is required for suchcharging due to a limited drive capability of the VBBGEN(n) 24 andVBBGEN(p) 26. Even if the capacitor NCAP is coupled to the substratePSUB with the capacitor PCAP coupled to the substrate NSUB at a timepoint earlier than time t1 when the capacitors NCAP, PCAP areincompletely charged as indicated in FIG. 13, the above-mentioned effectof the foregoing embodiments cannot be fully produced because the amountof voltage change in each of the substrates PSUB, NSUB is small.

In view of the above circumstances, the present embodiment uses a uniquecapacitor for each block, and assigns different priorities to the blocksto charge the capacitors in order from the highest-priority block to thelowest. Therefore, only high-priority blocks can be rapidly charged. Theelectrical charges of the substrates are regenerated to the capacitorsat the same time without regard to the priorities.

Leakage current in the standby state causes a problem for somefunctional blocks on the chip and causes no problem for the otherfunctional blocks. The leakage current in the standby state causes noproblem for functional blocks that have a small overall leakage currentor do not need to enter the standby state because power can be shut offfrom an entire functional block although the leakage current is large.

The functional blocks shown in FIG. 17 have the characteristicsdescribed below.

The SRAM 14 has a large leakage current because it stores data andcannot be shut off from power. The flash memory 16 can be shut off frompower. The CPU core 12 includes a small amount of SRAM, but the majorpart of the CPU core 12 can be shut off from power. The timer 18 has asmall leakage current.

As is obvious from the above, the highest priority is assigned to thereduction of leakage current of the SRAM 14. Consequently, the capacitorNCAP11 coupled to the SRAM 14 should be preferentially charged.

FIG. 18 is a diagram illustrating temporal voltage changes that occur inthe node vcapn when a capacitor common to all blocks is charged.

More specifically, FIG. 18 illustrates voltage changes in the node vncapcoupled to one end of the capacitor NCAP when the capacitor NCAPincluded in a bias control circuit coupled to all the functional blocksis charged.

FIG. 19 is a diagram illustrating temporal voltage changes that occur inthe node vncap when capacitors for individual functional blocks aresequentially charged.

More specifically, FIG. 19 illustrates voltage changes in the node vncapcoupled to one end of the capacitor NCAP when the capacitor NCAPincluded in a bias control circuit coupled to a selected functionalblock is charged. The functional blocks are sequentially selected inorder from the highest-priority functional block to the lowest.

In general, the capacitance of the substrate of a SRAM is not higherthan 1/10 the capacitance of a whole chip. Therefore, the time requiredfor charging a capacitor coupled to a SRAM according to the presentembodiment can be reduced to 1/10 or less the time required for charginga capacitor commonly coupled to all the blocks.

FIG. 20 is a diagram illustrating an exemplary cross-sectional structureof a substrate for applying different bias voltages to the functionalblocks within the chip.

A P-type support substrate is used. Plural deep n-wells separated intothe individual functional blocks are disposed in the P-type supportsubstrate.

A voltage of (VDD+VBB) is applied to a deep n-well in a left block shownin FIG. 20. The back bias voltages applied to the NMOS transistor andthe PMOS transistor are (−VBB) and (VDD+VBB), respectively.

A voltage of VDD is applied to a deep n-well in a right block shown inFIG. 20. The back bias voltages applied to the NMOS transistor and thePMOS transistor are 0 V and VDD, respectively.

As described above, different back bias voltage regions can be set onthe same support substrate by using the deep n-wells.

Sixth Embodiment

FIG. 21 is a diagram illustrating the bias control circuit according toa sixth embodiment of the present invention.

The sixth embodiment provides a unique capacitor CAP for each functionalblock in the chip, and includes a common substrate wiring CSUB andswitches SWN4 i (i=1 to n). The common substrate wiring CSUBshort-circuits substrates in the individual blocks. The switches SWN4 iare disposed between the substrates PSUBi and the common substratewiring CSUB. Although the chip includes various functional blocks, acertain functional block may not be used at all. In such an instance, anunused functional block is shutoff from power, but the capacitance ofthe substrate remains. Therefore, the substrate capacitance of an unusedfunctional block is used as a capacitor CAP.

As shown in FIG. 21, a switch SWN4 i couples a substrate PSUBi in anunused functional block to the common substrate wiring CSUB. The unusedfunctional block is then used as a capacitor for a functional blockhaving high priority for back bias voltage control.

As described above, when the substrate capacitance of an unusedfunctional block is used as a capacitor, the back bias voltage can becontrolled at high speed by using the capacitor even if no dedicatedcapacitor for back bias voltage is available. This also saves the chiparea for a dedicated capacitor for back bias voltage.

The bias control circuits NLk are capable of selectively coupling thesubstrate of the MOSFET included in the associated circuit block to theVBBGEN(n) 24 or to the ground GND or placing the substrate in anopen-circuit state.

The switches SWN4 i are capable of selectively coupling or uncoupling anode Lk in the path to the substrate of the MOSFET included in theassociated circuit block to or from the common substrate wiring CSUB.

In the operating mode of a used circuit block, the bias control circuit(designated as NXi) of an unused circuit block uses a switch SWN3 i tosupply the output of the VBBGEN(n) to the PSUBi, which is the substrateof the MOSFET in the unused circuit block.

When a used circuit block transitions from the operating mode to thestandby mode, the bias control circuit (designated as NXj) of the usedcircuit block uses the switches SWN4 i, SWN4 j and the common substratewiring CSUB to supply the electrical charge stored in the PSUBi to asecond substrate that is the substrate of the MOSFET included in theused circuit block.

FIG. 22 is a timing diagram illustrating an operation of the circuitshown in FIG. 21. It is assumed that the output voltage of the VBBGEN(n)26 is always (−VBB), and that a substrate PSUB1 is equal in capacitanceto a substrate PSUBn. It is also assumed that the SRAM 14 is a usedcircuit block, and that the timer 18 is an unused circuit block. That isto say, i=1 and j=n.

It is assumed that, in an initial state at time t0, the voltage vpsub1of the substrate PSUB1 and the voltage vpsubn of the substrate PSUBn areboth equal to a ground voltage of 0 V.

When the CPU core 12 sets the switch SWN31 to c1, the switch SWN41 toy1, the switch SWN3 n to bn, and the switch SWN4 n to yn, the VBBGEN(n)24 is coupled to the substrate PSUBn, and the substrate PSUB1 is coupledto the ground GND. During this period, the SRAM 14 is in the operatingstate, and the voltage of the substrate PSUB1 remains at 0 V. The timer18 is in a charging state so that the substrate PSUBn is graduallycharged to (−VBB). The time required for such charging is governed bythe maximum output current of the VBBGEN(n) 24.

When, at time t1, the voltage of the substrate PSUBn is (−VBB), theVBBGEN(n) 24 automatically stops to terminate charging.

At time t2, the semiconductor device receives an instruction for causingthe SRAM 14 to transition from the operating state to the standby state.In this instance, the CPU core 12 sets the switch SWN31 to a1, theswitch SWN41 to x1, the switch SWN3 n to an, and the switch SWN4 n toxn. This uncouples the VBBGEN (n) 24 from the substrate PSUBn, andcouples the substrate PSUB1 to the substrate PSUBn through the commonsubstrate wiring CSUB. The substrate PSUB1 is then rapidly charged by anegative electrical charge stored in the substrate PSUBn so that thevoltage vpsub1 of the substrate PSUB1 and the voltage vpsubn of thesubstrate PSUBn are both equal to (−½) *VBB. The time required for suchan operation is determined by the on-resistance of the switch SWN41 andswitch SWN4 n and the capacitance of the substrate PSUB1 and substratePSUBn. However, the substrate PSUB1 can be charged more rapidly bysufficiently decreasing the on-resistance than when the substrate PSUB1is charged by the VBBGEN(n) 24.

At time t3, the CPU core 12 further sets the switch SWN31 to b, theswitch SWN41 to y1, the switch SWN3 n to an, and the switch SWN4 n toyn. This couples the VBBGEN(n) 24 to the substrate PSUB1 and graduallycharges the substrate PSUB1 to (−VBB).

When, at time t4, the voltage of the substrate PSUB1 is (−VBB), theVBBGEN(n) 24 automatically stops to terminate charging.

At time t5, the semiconductor device receives an instruction for causingthe SRAM 14 to transition from the standby state to the operating state.The CPU core 12 sets the switch SWN31 to a1, the switch SWN41 to x1, theswitch SWN3 n to an, and the switch SWN4 n to xn. This recouples thesubstrate PSUB1 to the substrate PSUBn. The negative electrical chargeof the substrate PSUB1 is then rapidly regenerated to the substratePSUBn so that the voltage vpsub1 of the substrate PSUB1 and the voltagevpsubn of the substrate PSUBn are both equal to (−¾)*VBB. The timerequired for such a regeneration operation is determined by theon-resistance of the switch SWN41 and switch SWN4 n and the capacitanceof the substrate PSUB1 and substrate PSUBn. Therefore, the regenerationoperation is performed at the same high speed as the operation performedat time t2.

At time t6, the CPU core 12 sets the switch SWN31 to c1, the switchSWN41 to y1, the switch SWN3 n to bn, and the switch SWN4 n to yn. Thisrapidly discharges the substrate PSUB1 to 0 V. The time required forsuch a discharge operation is determined by the on-resistance of theswitch SWN41 and switch SWN4 n and the capacitance of the substratePSUB1. Therefore, the discharge operation is also performed at highspeed. The substrate PSUBn is coupled to the VBBGEN(n) 24 and graduallycharged again to (−VBB).

When, at time t7, the substrate PSUB1 is discharged to 0 V, the circuitblocks are placed in the operating state.

When, at time t8, the voltage of the substrate PSUBn is (−VBB), theVBBGEN(n) 24 automatically stops to terminate charging.

The state at time t8 is the same as the state at time t1. Subsequently,a cycle formed of states at time t1 to time t8 is repeated forprocessing purposes.

While the present invention made by its inventors has been described indetail in terms of particular embodiments, the present invention is notlimited to the foregoing embodiments. It is to be understood by thoseskilled in the art that various modifications can be made withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A semiconductor device comprising: a circuitblock including at least a MOSFET, and having two operating modes,namely, an operating mode and a standby mode; a voltage generationcircuit configured to generate and output a predetermined outputvoltage; and a bias control circuit coupled between the circuit blockand the voltage generation circuit and configured to receive thepredetermined output voltage from the voltage generation circuit, andfurther configured to store an electrical charge while the circuit blockis in the operating mode, supply the stored electrical charge to asubstrate of the MOSFET included in the circuit block when the circuitblock transitions from the operating mode to the standby mode, andsubsequently supply the predetermined output voltage to the substrate ofthe MOSFET, the bias control circuit including at least one capacitorconfigured to selectively couple to an output of the voltage generationcircuit in the operation mode and the standby mode, wherein thepredetermined output voltage is a back bias voltage of the substrate ofthe MOSFET in the standby mode.
 2. The semiconductor device according toclaim 1, wherein the circuit block includes an n-type MOSFET and ap-type MOSFET, wherein the voltage generation circuit includes: a firstportion of the voltage generation circuit that outputs a firstpredetermined output voltage, the first predetermined output voltagebeing the back bias voltage of the substrate of the n-type MOSFET in thestandby mode; a second portion of the voltage generation circuit thatoutputs a second predetermined output voltage, the second predeterminedoutput voltage being the back bias voltage of the substrate of thep-type MOSFET in the standby mode, wherein the bias control circuitincludes: a first portion of the bias control circuit that stores anelectrical charge while the circuit block is in the operating mode,supplies the stored electrical charge to the substrate of the n-typeMOSFET included in the circuit block when the circuit block transitionsfrom the operating mode to the standby mode, and subsequently suppliesthe first predetermined output voltage to the substrate of the n-typeMOSFET; and a second portion of the bias control circuit that stores anelectrical charge while the circuit block is in the operating mode,supplies the stored electrical charge to the substrate of the p-typeMOSFET included in the circuit block when the circuit block transitionsfrom the operating mode to the standby mode, and subsequently suppliesthe second predetermined output voltage to the substrate of the p-typeMOSFET.
 3. The semiconductor device according to claim 1, wherein thebias control circuit supplies an electrical charge stored in thesubstrate of the MOSFET to the capacitor when the circuit blocktransitions from the standby mode to the operating mode, andsubsequently couples the substrate of the MOSFET to a supply source of asecond voltage, the second voltage being the back bias voltage of thesubstrate of the MOSFET in the operating mode.
 4. The semiconductordevice according to claim 3, wherein the bias control circuit furtherincludes a first switch and a second switch, the capacitor beingdisposed between a first node and the supply source of the secondvoltage, the first switch being configured to selectively open or closea path between the first node and the output of the voltage generationcircuit, the second switch being configured to selectively couple thesubstrate of the MOSFET to the voltage generation circuit, to the firstnode, or to the supply source of the second voltage.
 5. Thesemiconductor device according to claim 4, wherein, in the operatingmode, the first switch opens the path, and the second switch couples thesubstrate of the MOSFET to the supply source of the second voltage,wherein, upon receiving an instruction for transitioning from theoperating mode to the standby mode, the first switch closes the path,and the second switch first couples the substrate of the MOSFET to thefirst node and then couples the substrate of the MOSFET to the voltagegeneration circuit, and wherein, upon receiving an instruction fortransitioning from the standby mode to the operating mode, the firstswitch closes the path and the second switch couples the substrate ofthe MOSFET to the first node, and subsequently, the first switch opensthe path and the second switch couples the substrate of the MOSFET tothe supply source of the second voltage.
 6. The semiconductor deviceaccording to claim 3, wherein the bias control circuit further includesa first switch, a second switch, and a third switch, the capacitor beingdisposed between a first node and a second node, the first switch beingconfigured to selectively couple the first node to the output of thevoltage generation circuit or to the supply source of the second voltageor placing the first node in an open-circuit state, the second switchbeing configured to selectively couple the substrate of the MOSFET tothe voltage generation circuit, to the first node, or to the supplysource of the second voltage, the third switch being configured toselectively couple the second node to a supply source of a third voltageor to the supply source of the second voltage.
 7. The semiconductordevice according to claim 6, wherein, in the operating mode, the firstswitch couples the first node to the supply source of the secondvoltage, the second switch couples the substrate of the MOSFET to thesupply source of the second voltage, and the third switch couples thesecond node to the supply source of the third voltage, wherein,subsequently, the first switch is placed in the open-circuit state, thesecond switch keeps the substrate of the MOSFET coupled to the supplysource of the second voltage, and the third switch couples the secondnode to the supply source of the second voltage, wherein, upon receivingan instruction for transitioning from the operating mode to the standbymode, the first switch remains in the open-circuit state, the secondswitch couples the substrate of the MOSFET to the first node, and thethird switch keeps the second node coupled to the supply source of thesecond voltage, wherein, subsequently, the first switch remains in theopen-circuit state, the second switch couples the substrate of theMOSFET to the voltage generation circuit, and the third switch keeps thesecond node coupled to the supply source of the second voltage, wherein,upon receiving an instruction for transitioning from the standby mode tothe operating mode, the first switch remains in the open-circuit state,the second switch couples the substrate of the MOSFET to the first node,and the third switch keeps the second node coupled to the supply sourceof the second voltage, wherein, subsequently, the first switch couplesthe first node to the supply source of the second voltage, the secondswitch couples the substrate of the MOSFET to the supply source of thesecond voltage, and the third switch couples the second node to thesupply source of the third voltage, and wherein, subsequently, the firstswitch is placed in the open-circuit state, the second switch keeps thesubstrate of the MOSFET coupled to the supply source of the secondvoltage, and the third switch couples the second node to the supplysource of the second voltage.
 8. The semiconductor device according toclaim 6, wherein, in the operating mode, the first switch couples thefirst node to the supply source of the second voltage, the second switchcouples the substrate of the MOSFET to the supply source of the secondvoltage, and the third switch couples the second node to the supplysource of the third voltage, wherein, subsequently, the first switchcouples the first node to the output of the voltage generation circuit,the second switch keeps the substrate of the MOSFET coupled to thesupply source of the second voltage, and the third switch keeps thesecond node coupled to the supply source of the third voltage, wherein,subsequently, the first switch is placed in the open-circuit state, thesecond switch keeps the substrate of the MOSFET coupled to the supplysource of the second voltage, and the third switch couples the secondnode to the supply source of the second voltage, wherein, upon receivingan instruction for transitioning from the operating mode to the standbymode, the first switch remains in the open-circuit state, the secondswitch couples the substrate of the MOSFET to the first node, and thethird switch keeps the second node coupled to the supply source of thesecond voltage, wherein, subsequently, the first switch remains in theopen-circuit state, the second switch couples the substrate of theMOSFET to the voltage generation circuit, and the third switch keeps thesecond node coupled to the supply source of the second voltage, wherein,upon receiving an instruction for transitioning from the standby mode tothe operating mode, the first switch couples the first node to theoutput of the voltage generation circuit, the second switch couples thesubstrate of the MOSFET to the supply source of the second voltage, andthe third switch couples the second node to the supply source of thethird voltage, and wherein, subsequently, the first switch is placed inthe open-circuit state, the second switch keeps the substrate of theMOSFET coupled to the supply source of the second voltage, and the thirdswitch couples the second node to the supply source of the secondvoltage.
 9. The semiconductor device according to claim 2, wherein thefirst portion of the bias control circuit includes a first capacitorconfigured to selectively couple to an output of the first portion ofthe voltage generation circuit, and wherein the second portion of thebias control circuit includes a second capacitor configured toselectively couple to an output of the second portion of the voltagegeneration circuit.
 10. A semiconductor device comprising: a pluralityof circuit blocks having two operating modes, namely, an operating modeand a standby mode; a voltage generation circuit that is commonlyprovided for the circuit blocks and which is configured to generate andoutput a predetermined output voltage; and a plurality of bias controlcircuits that are provided respectively for the circuit blocks, andconfigured to receive the predetermined output voltage from the voltagegeneration circuit, wherein the bias control circuits each store anelectrical charge supplied from the voltage generation circuit when theassociated circuit block in the operating mode is selected, supplies thestored electrical charge to a substrate of a MOSFET included in theassociated circuit block when the associated circuit block transitionsfrom the operating mode to the standby mode, and subsequently suppliesthe predetermined output voltage to the substrate of the MOSFET, whereinthe circuit blocks are each assigned different priorities and selectedaccording to the assigned priorities, and wherein the predeterminedoutput voltage is a back bias voltage of the substrate of the MOSFET inthe standby mode.
 11. A semiconductor device comprising: a plurality ofcircuit blocks having two operating modes, namely, an operating mode anda standby mode; a voltage generation circuit that is commonly providedfor the circuit blocks and which is configured to generate and output apredetermined output voltage; and a plurality of bias control circuitsthat are provided respectively for the circuit blocks, and configured toreceive the predetermined output voltage from the voltage generationcircuit, wherein a bias control circuit for an unused circuit blocksupplies the predetermined output voltage of the voltage generationcircuit to a first substrate in the operating mode of a used circuitblock, the first substrate being a substrate of a MOSFET in the unusedcircuit block, and wherein a bias control circuit for a used circuitblock supplies an electrical charge stored in the first substrate to asecond substrate when the used circuit block transitions from theoperating mode to the standby mode, and subsequently supplies thepredetermined output voltage of the voltage generation circuit to thesecond substrate, the second substrate being a substrate of a MOSFET inthe used circuit block.
 12. The semiconductor device according to claim11, further comprising: a common wiring that is commonly provided forthe circuit blocks, wherein the bias control circuits each include afirst switch and a second switch, the first switch being configured toselectively couple a substrate of a MOSFET included in the associatedcircuit block to the voltage generation circuit or to a supply source ofa second voltage or placing the substrate of the MOSFET in anopen-circuit state, the second switch being configured to selectivelycouple or uncouple the common wiring to or from a node in a path betweenthe first switch and the substrate of the MOSFET, and wherein the secondvoltage is a back bias voltage of the substrate of the MOSFET in theoperating mode.
 13. The semiconductor device according to claim 12,wherein, when the used circuit block is in the operating mode, in thebias control circuit for the unused circuit block, the first switchcouples the substrate of the MOSFET to the voltage generation circuitwhereas the second switch refrains from coupling the node to the commonwiring, or in the bias control circuit for the used circuit block, thefirst switch couples the substrate of the MOSFET to the supply source ofthe second voltage whereas the second switch refrains from coupling thenode to the common wiring, and wherein, when the used circuit blockreceives an instruction for transitioning from the operating mode to thestandby mode, in the bias control circuit for the unused circuit block,the first switch is placed in an open-circuit state and the secondswitch couples the node to the common wiring, or in the bias controlcircuit for the used circuit block, the first switch is placed in theopen-circuit state whereas the second switch couples the node to thecommon wiring and subsequently the first switch couples the substrate ofthe MOSFET to the voltage generation circuit whereas the second switchrefrains from coupling the node to the common wiring.